Commands

Commands#

This page explains how linux runs commands.

Path#

This page explains how Linux determines what to execute when you run any command in it. There is a special environment variable, PATH, that specifies where to search for programs to be executed when a command is invoked from the shell. Each path is separated by a colon (:). When any command is typed, the system checks all paths listed in PATH for a corresponding file. When it finds the appropriate file, it executes it.

Extending#

PATH can be extended using the command export PATH=$PATH:<new section of path>, which modifies the PATH environment variable by appending a new section to it.

The following cell creates a new script that prints Hello, World! to the standard output and makes it executable.

[ -d /tmp/path_extending/ ] && rm -r /tmp/path_extending
mkdir /tmp/path_extending
cd /tmp/path_extending

cat << EOF > show_me
echo "Hello world"
EOF

chmod +x show_me

You can easily run it by specifying a direct path.

./show_me

Hello world

But after adding the appropriate path to the PATH variable, you can run the command from any path you like. Following cells show the thing.

export PATH=$PATH:/tmp/path_extending

cd ~/
show_me

Hello world

Find path (which)#

Sometimes it’s useful to retrieve the path of a command available in the shell. This can be done using the which <command> syntax. Use which -a <command> to display all paths to a given command.

The following cell applies the which command to the ls command.

which ls

/usr/bin/ls

To guarantee that the result is correct, let’s copy the ls command and name it hello. As a result, hello should behave just like ls.

cp /usr/bin/ls /usr/bin/hello

The following cell invokes hello /tmp, and the result looks just like the files available in the /tmp folder.

hello /tmp

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vscode-ipc-1a675766-8108-4b19-a9c6-b2ddff78e404.sock
vscode-ipc-791e9e66-9310-45f3-a55e-f91811b29434.sock
vscode-ipc-9415dfa0-4e3c-48bd-8640-fe0f3fbed7f4.sock
vscode-ipc-fb102fc3-61d7-42ae-9e80-e12b7a136bf5.sock
vscode-remote-containers-0da91551-794b-4873-832f-b827f3d0b9b7.js
vscode-remote-containers-83e67190-1f4d-4195-a09f-62dbe1f51e94.js
vscode-remote-containers-d6916203-e4d1-40f2-9ef2-1135c4ff3e04.js
vscode-remote-containers-ipc-0da91551-794b-4873-832f-b827f3d0b9b7.sock
vscode-remote-containers-ipc-83e67190-1f4d-4195-a09f-62dbe1f51e94.sock
vscode-remote-containers-ipc-d6916203-e4d1-40f2-9ef2-1135c4ff3e04.sock
vscode-remote-containers-server-0da91551-794b-4873-832f-b827f3d0b9b7.js
vscode-remote-containers-server-83e67190-1f4d-4195-a09f-62dbe1f51e94.js
vscode-remote-containers-server-d6916203-e4d1-40f2-9ef2-1135c4ff3e04.js
vscode-ssh-auth-0da91551-794b-4873-832f-b827f3d0b9b7.sock
vscode-ssh-auth-83e67190-1f4d-4195-a09f-62dbe1f51e94.sock
vscode-ssh-auth-d6916203-e4d1-40f2-9ef2-1135c4ff3e04.sock

You can even find the path to the which command itself.

which which

/usr/bin/which

The next cell creates the file /tmp/second_ls/ls and adds it to PATH to demonstrate how which behaves in this case.

mkdir /tmp/second_ls
touch /tmp/second_ls/ls
export PATH=$PATH:/tmp/second_ls/

By simply invoking which ls, it shows only one path to the original ls, as it appears first in PATH.

which ls

/usr/bin/ls

But which -a ls shows both pathes.

which -a ls

/usr/bin/ls
/bin/ls
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